Receptor-like protein-tyrosine phosphatases (RPTPs) play important roles in regulating intracellular processes. We have been investigating the regulation and function of RPTPmu, a receptor-like PTP related to the Ig superfamily of cell adhesion molecules. Recently, the crystal structure of a dimer of the membrane proximal domain of RPTPalpha (RPTPalpha D1) was described (Bilwes, A. M., den Hertog, J., Hunter, T., and Noel J. P. (1996) Nature 382, 555-559). Within this crystal structure, the catalytic site of each subunit of the dimer is sterically blocked by the insertion of the N-terminal helix-turn-helix segment of the dyad-related monomer. It was proposed that dimerization would lead to inhibition of catalytic activity and may provide a paradigm for the regulation of the RPTP family. We have determined the crystal structure, to 2.3 A resolution, of RPTPmu D1, which shares 46% sequence identity with that of RPTPalpha D1. Although the tertiary structures of RPTPalpha D1 and RPTPmu D1 are very similar, with a root mean square deviation between equivalent Calpha atoms of 1.1 A, the quaternary structures of these two proteins are different. Neither the catalytic site nor the N-terminal helix-turn-helix segment of RPTPmu D1 participates in protein-protein interactions. The catalytic site of RPTPmu D1 is unhindered and adopts an open conformation similar to that of the cytosolic PTP, PTP1B (Barford, D., Flint, A. J., and Tonks, N. K. (1994) Science 263, 1397-1404). We propose that dimerization-induced modulation of RPTP activity may not be a general feature of this family of enzymes.